Lithium-ion (“Li-ion”) batteries are extensively used for energy storage applications. These applications include powering electric vehicles (“EVs”) and personal electric devices, such as laptops computers, digital music players, smart phones, and so forth. Li-ion batteries are particularly advantageous in these application due to their high energy density, high operational voltage, and low self-discharge rate. However, despite their widespread use and growing popularity, serious technical challenges remain in the use of Li-ion cells. These challenges include range per charge, charging time, cost, safety, and most importantly, cell lifetime. These challenges are especially pronounced in EV applications where long-term cycling and lifetimes of 10-15 years are expected.
To increase the lifetime of a Li-ion cell, degradation and aging mechanisms must first be comprehensively understood. Growth of a solid electrolyte interface (SEI) over time and the phenomenon known as lithium plating are considered the most important degradation modes. SEI is a passivation layer that forms on the surface of the active particles due to unwanted lithium reactions. Lithium plating is a cathodic reaction where lithium ions are reduced from ionic lithium to metallic lithium on the surface of the active particles of the anode underneath the SEI layer. While part of the metallic lithium may be stripped away during rest and discharge, a portion referred to as “dead lithium” cannot be removed. This provokes safety issues, such as dendrite growth, in addition to a reduction of the overall cell capacity. These degradation modes also cause cell capacity fade and increase the internal resistance of the cell resulting in a shorter lifetime.
Lithium plating and SEI growth both result from parasitic and undesired reactions in the negative electrode. The rates of these reactions strongly depend on the cycling conditions and the design of the cell. These rates also tend to increase when increasing the charging rate of the cell. SEI growth and lithium plating have been extensively studied and researchers have mainly suggested electrolyte additives to reduce the rates of the unwanted reactions. However, a higher charging rate is expected and necessary for next generation Li-ion battery applications in the state-of-the-art EV technology. Therefore, a more robust solution is required in the art.